Long Life Bearings for Steel Rolling MillsWTF® BearingsNSK’s newly developed Water TF (WTF®) bearings are the result ofcombining special steel-melting technology used for aircraft bearingmaterial, and our original, heat treatment technology. Our WTF(water-tough) bearings successfully resist premature flaking underseverely contaminated conditions of water-infiltrated greaselubrication and powder debris (iron-oxide) contamination.Furthermore, our WTF bearings have a bearing life that is threetimes longer than our conventional bearings.

21

WTF® BearingsA new product that responds to customer needsfor longer life in harsh environments.Using NSK technology, we created WTF bearings to overcomepremature flaking that results from conditions of water-infiltratedgrease lubrication and powder debris or particulate contamination. WTF bearings achieve a service life three times longer under severeoperating conditions.

Long Life WTF® Bearings for Steel Rolling MillsWTF® Bearings

Premature flaking is initiated by dents that are generatedon bearing rolling surface areas, which are caused byparticulate contamination. When the bearings areoperated under such conditions with water-infiltratedgrease lubrication, the strength of bearing steel issignificantly weakened. This ultimately leads to aphenomenon called corrosion fatigue (Photo 1). Bycombining material and heat treatment technologies intowhat NSK calls Water TF (water-tough) technology, wecan drastically prevent the onset of corrosion fatigue.

NSK’s Water TF technology includes many proprietarytechnologies that have been patented (below) with manymore pending approval.

USP6165289, GB2311998, USP5256213,USP5298323, JP2128328, USP4904094,GB2209058

Cracks lead to flaking and ultimately corrosion fatigue

1. Technology

Photo 1. Water TF technology consists of an ultra-high clean-material technology for inhibiting thegeneration of cracks, and a chemical component design technology for restricting infiltration of water intoany cracks that developed.

0.2 mm

Bearings used under contaminated lubrication conditionssuffer from indentations that are generated on theraceways by foreign particles, which initiate surfaceoriginated flaking.Stress concentration points around the edges of thesurface indentations create cracks around theindentations, which progress into larger cracks that leadto flaking. Contact stress concentrations at the edges ofan indentation (Figure 2) can be expressed by[P/P0∝(r/c)-0.24], which is a function of r/c. This suggests

that the larger the r/c, the smaller the stressconcentration, which means an ultimately longer life.NSK’s research on bearing material confirms that as theamount of retained austenite increases, the r/c valuebecomes larger, and the reduction of stressconcentrations is enhanced.Our patented TF technology incorporates this principlefor achieving longer life under contaminated lubricationconditions by maintaining optimum levels of retainedaustenite.

1.0

0.8

0.6

0.4

0.2

00 10 20 30 40 50 60

r/c

valu

e

Retained austenite%

x

Po Stress at Infinity

Defect ShoulderArbitrary Smooth

Profile E2

E1c c

rr

2b

Po

0 1.0 2.0 3.0 4.0 5.0

1.0

2.0

3.0

4.0

0

Xo

c

x/b

P/P

o va

lue

Figure 1. Relation between r/c value and levels of retained austenite

Figure 2. Stress concentrations at the edges of surface indentations[ Y.P.chiu Trans-asme, Ser-F, (1970) ]

2b: Range over which contact pressuredoes not act, mm

2c: Shoulder-to-shoulder width of surfacedent, mm

E1, E2: Elastic modulus, MPa

P0: Contact pressure without dent, MPa

P: Contact pressure with dent, MPa

r : Curvature around a surface indentation, mm

NSK developed its Water TF technology by incorporating components of TF technologyfor enhanced service life under contaminated lubrication conditions.

Nonmetallic inclusions (Photo 2) develop cracks below the bearing surface (Photo 3). Water in the greaselubricant infiltrates the cracks, and weakens the bearing material. Using a special steel-melting technologyused for aircraft engine bearing material, we were able to restrict crack occurrence by reducing theamount of non-metallic inclusions in the raceway surface.

In order to inhibit the initial development of cracks along grain boundaries, we worked to achieve strongergrain boundaries (Photo 4) by developing an optimum alloy balance for bearing material.

0.5 mm

0.5 mm

Photo 2. Non-metallic inclusion Photo 3. Cracks develop from below the surface

Photo 4. Cracks along grain boundary

A

Rolling surface

43

Measures againstparticulate contamination

Improvement offoreign particle

resistance

Resistance tocrack generationcaused bycorrosion fatiguedue to waterinfiltration

Resistance tocrack propagationcaused by corrosionfatigue due towaterinfiltration

Measures againstwater infiltration

Long Life

Surface originated flaking(for water-infiltrated

environments)

Subsurface originated flaking(for clean environments)

High cleanliness(mass-produced

melting)

Ultra-highcleanliness

(special melting)

Chemicalcomponent

designtechnology

Control ofretainedaustenite

Special heattreatment

Sealedfour-row tapered

roller bearing(E-CAPA series)

T/M sealedclean bearing

Material

Z steel TF bearing

Hi-TF steel

Super TFbearing

Water TFbearing

Aircraftengine

bearings

UR bearing

EP steel

Bearing sealHeat treatment

Surface originated flaking(for particulate contaminated

lubrication environments)

WTF® Bearings

2. WTF technology among otherNSK long life technologies

3. Life of WTF bearings

Life, h

10 102 103 104

Cum

ulat

ive

failu

re r

ate,

%

1

5

10

50

90

9999.9

Water supply: 20 cc/7 hSize of fine iron powder particles:

max. 32 μmP/C: 0.25N=1 500 min–1

L10cal: 1 130 hGrease: ADREX

Conventionalmaterial WTF® material WTF® material

Cum

ulat

ive

failu

re r

ate,

%

Life (in tonnage of material rolled), 104 t

10 102 1031

5

10

50

90

9999.9

Conventionalmaterial

Figure 3. Test results of tapered roller bearing life undercontaminated lubrication conditions of grease, water,and fine iron powder simulating market conditions

254

12 44 22.24

127 127

3.3

3.3

3.8

1.5

124422.2497.52

416

362

254

φ

343.

052

φ

457.

098

φφ

Purpose

Challenge

Measures

Bearing life test results under contaminated lubricationconditions of grease, water, and fine iron powdersimulating the market are described in Figure 3. Trialusage results of actual cold rolling mills are described inFigure 4. Both results show a bearing life three timeslonger than that of conventional bearings.

Bearing type symbol

Example ofbearing number: WTF KVS3 4 3 4 5 5 1

Water TF

Bore diameter

Outside diameter

Figure 5. WTF bearing dimensions (example)

Figure 4. Trial usage results of WTF bearings in anactual cold rolling mill

65

Long Life WTF® Bearings for Steel Rolling Mills

NSK Ltd. has a basic policy not to export any products or technology designated as controlled items by export-related laws. When exporting the products in thisbrochure, the laws of the exporting country must be observed. Specifications are subject to change without notice and without any obligation on the part of themanufacturer. Every care has been taken to ensure the accuracy of the data contained in this brochure, but no liability can be accepted for any loss or damagesuffered through errors or omissions. We will gratefully acknowledge any additions or corrections.

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